This invention relates to electronic circuitry for reducing emissions of electromagnetic interference. More particularly, the invention relates to such circuitry using digital spread spectrum means.
Many electronic devices employ microprocessors or other digital circuits that require one or more clock signals for synchronization. A clock signal permits the precise timing of events in the microprocessor. Typical microprocessors may be supervised or synchronized by a free-running oscillator, such as driven by a crystal, an LC-tuned circuit, or an external clock source.
High performance, microprocessor-based devices using leading edge, high-speed circuits are particularly susceptible to generating and radiating electromagnetic interference (EMI). Undesirable EMI is generated when a clock electronic circuit produces a pulsed signal having a pronounced energy peak, including at harmonic frequencies. The spectral components of the EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of the clock circuit. These high-power harmonic electromagnetic pulses can interfere with other carrier signals at higher frequencies, such as radio waves. Accordingly, many regulatory agencies, such as the Federal Communications Commission in the United States, have established testing procedures and maximum allowable electromagnetic emissions levels for such devices.
To comply with such government limits on EMI emissions, costly suppression measures or extensive shielding have been used. Other approaches for reducing EMI have included careful routing of signal traces on printed circuit boards to minimize loops and other potentially radiating structures. Unfortunately, such an approach often leads to more expensive multi-layer circuit boards with internal ground planes. In addition, greater engineering effort must go into reducing EMI emissions.
It is also known to lower the pronounced energy peak and still accomplish the desired clock function by using spread spectrum techniques that modulate the clock slowly to spread the energy of the of the signal over a broader portion of the spectrum about either side of the original peak.
Several spread spectrum modulation techniques have been employed in the past in an attempt to minimize the emission of magnetic interference from electronic devices, including printers. Many of the latest spread spectrum modulation techniques employ a phase-locked loop (PLL) in conjunction with a voltage-controlled oscillator (VCO). PLL-based spread spectrum systems rely on analog modulation methods and do not operate reliably when the fundamental clock input to the system is rapidly turned on and off. A separate waveform having a specific profile is overlaid on the waveform of the clock generator, causing a frequency shift during the generation of each clock pulse. A PLL-based system can fail to adequately reduce EMI if the modulated frequency is changed too slowly. Further, PLL-based modulator frequency output can drift due to comparator phase delays, resulting in system jitter and failing to provide a modulated signal with sufficient quality to control or synchronize other elements of the dependent microprocessor or digital circuit.
Following is a review of several relevant spread spectrum modulation techniques found in the prior art.
Hardin et al, Spread Spectrum Clock Generator And Associated Method, U.S. Pat. No. 5,488,627 (Jan. 30, 1996) provides a slight variation on the aforementioned PLL-based spread spectrum modulator systems. In Hardin""s system, an analog method is used to sweep up and down the reference clock signal frequency to provide the desired modulation. Hardin""s method likewise employs a PLL as part of its circuitry. In another patent, Hardin, Spread Spectrum Clock Generator, U.S. Pat. No. 5,631,920 (May 20, 1997), describes another PLL-based spread spectrum modulation approach which is to similar to Hardin""s earlier patent, but simply uses a different analog method to sweep up and down the reference clock signal frequency.
Puckette et al, Digital Method And Apparatus For Reducing EMI Emissions In Digitally Clocked Systems, U. S. Pat. No. 5,736,893 (Apr. 7, 1998) describes a complex spread spectrum signal modulation system requiring at least one PLL to regenerate the needed frequencies to modulate the clock signal. In a system requiring rapid initialization, Puckette""s system would not operate effectively due to the delays associated with ramp-up and synchronization between the input clock signal and the associated reference signal.
Another PLL-based system, Bassetti et al, EMI Reduction For A Flat-panel Display Controller Using Horizontal-line Based Spread Spectrum, U. S. Pat. No. 5,757,338 (May 26, 1998) is directed to a spread spectrum modulator for driving a flat panel display and CRT. In Bassetti""s system, the primary purpose is to minimize distortion of displayed images when both a CRT and a flat panel display are used concurrently and in close proximity. A horizontal clock counter is used to modulate the primary reference clock frequency in conjunction with analog components, including digital to analog converters and a voltage-controlled oscillator (VCO). Bassetti""s is complex and difficult to adjust for application to varying types of devices other than CRT""s and flat panel displays.
Knierim, Spread Spectrum Phase-locked Loop Clock Generator With VCO Driven By A Symmetrical Voltage Ramp Signal, U.S. Pat. No. 5,659,587 (Aug. 19, 1997) teaches a similar spread spectrum modulation system dependent upon an analog VCO and PLL. Bland, Circuit For Generating A Spread Spectrum Clock, U.S. Pat. No. 5,610,955 (Mar. 11, 1997) also teaches an analog VCO-based clock modulator.
Each of the above systems include inherent PLL-based delays which would preclude their use in systems, such as printers, where the primary input clock is repeatedly turned on and off.
Accordingly, a need exists for a simple yet effective electronic apparatus to reduce emissions of electromagnetic interference from microcircuits using clock pulses to regulate their operation. Additionally, a need exists for such an apparatus that is capable of operating during rapid initialization and shutdown of the microcircuit, such as that used in laser printers.
The invention provides a simple, flexible, and inexpensive device and method for reducing emissions of electromagnetic interference through the use of a novel electronic circuit. Further, the invention provides such a circuit that is operable even where the clock signal to the device is rapidly turned on and off. The spread spectrum system generates the spread spectrum output clock signal by parsing, sampling, delaying, and reaggregating the various frequency components of the original clock signal. The digital modulation, or spread spectrum simulation, reduces the spectral amplitude of the EMI components at each harmonic of the clock when compared to the spectrum of the same clocking signal without such digital modulation.
In a most basic configuration, the electronic circuit of the invention employs a primary non-modulated clock signal which acts as both the source of the output digitally modulated clock signal and also drives a standard flip-flop. The flip-flop actuates an MOS circuit that controls either the operation of a Resistor-Capacitor (xe2x80x9cRCxe2x80x9d) timing circuit or delay line. The RC timing circuit or delay line detunes the clock signal from the desired nominal frequency. By selecting first the non-modulated clock signal and then the delayed signal, a first signal is generated having a frequency at the clock signal, and, a second signal is generated which is displaced slightly from the first signal, but still at the clock frequency. When aggregated, a digitally modulated output signal is produced. Detuning of the clock signal allows the energy of the clock signal to be sufficiently spread over a broader frequency band to reduce the EMI at the primary clock frequency, while still providing desired clock function at the desired nominal frequency for synchronization of various elements of the microcircuit. By repeatedly switching the RC circuit or delay line in and out of the non-modulated clock signal path, a variety of samples of the non-modulated clock signal may be obtained and later aggregated to provide an output clock signal having an approximately equivalent nominal frequency, yet lacking the high energy harmonic pulses which would violate EMI restrictions.
A more scalable preferred embodiment of the invention includes a waveform or clock signal generation means, such as an oscillator, that drives an n input multiplexer, either directly, or, through nxe2x88x921 delay lines. As a result, multiple clock signals at varying frequencies may be created to spread the energy of the clock signal over a broader spectrum. Thus, the electromagnetic energy of the clock signal is spread on either side of its original energy peak, thus producing a modulated output clock signal. The modulated output clock signal has a lowered energy peak but retains a nominal frequency equivalent to that of the fundamental frequency of the primary clock signal. Consequently, the circuit being driven, regulated, synchronized or otherwise managed by the modulated clock signal still operates reliably. Hence, by lowering the output signal energy peak at the fundamental frequency and harmonics of that frequency, the individual electronic device is able to meet proscribed FCC regulations.
The invention particularly lends itself to uses, such as in a high-speed laser printer engine, where it is desirable to have the electronic device quickly, and repeatedly, initialize and shutdown. Conventional PLL-based spread spectrum modulators are incapable of performing in a system having such instantaneous start and stop requirements. The invention eliminates the need for the modulation circuit to reach a stable state or ramp up, as required with a PLL-based modulator. Consequently, systems which use the spread spectrum system of the present invention are able to respond more efficiently to changing operating conditions without exceeding EMI restrictions.